I thought of this scenario a few months back. Probably too ambitious, but still seems worth discussing.
In my Russian America timeline, the Empire of Japan would be among the Allied Powers and (therefore) would continue to hold onto the Korean Peninsula until the 1980s (at which point they become independent). Throughout the Cold War, Japanese settlers would (no doubt) begin to populate Korea and change the demographics.
For this scenario, Koreans would still make up the majority of the peninsula, with Japanese settlers dominating the south. When Korea moves to become independent, the Japanese settlers may oppose this and wish to remain with Japan.
This is where the "Northern Ireland" notion comes into play, in which Japan allowed Korea to become independent, but with a catch.
My personal thought is that such an exclave would be within the historic Gyeongsang Province region, which is the closest point to Japan and would (no doubt) be a key area under Japanese control.